The present invention relates to the measurements of disturbances in a television signal, and more particularly to an in-service cable television (CATV) hum measurement technique for automatically determining hum related to powerline frequencies and low frequency disturbances related to the video signal.
Hum and low frequency disturbances (LFD) are undesired signal modulations related to powerline or video vertical sync frequencies. Variations in the visual carrier amplitude at the powerline frequency or its harmonics are known as "hum". It is difficult to determine video-related disturbances and hum when a video channel is in-service. Consequently a continuous wave (CW) signal is usually substituted for an in-service video signal carrier, but only hum alone is measured. This is an undesirable limitation since the channel is taken off the air to measure hum and it does not include LFD. The real problem in attempting to measure hum at 60 Hz and LFD on an in-service, active video channel is that the vertical sync pulses are present at 59.94 Hz. It is typically not possible to separate relatively weak powerline hum and LFD signals from the large vertical sync signal present only 0.06 Hz away.
A prior manual method for measuring hum and LFD is set out in the Application Note No. 26W-7043, published by Tektronix, Inc. of Wilsonville, Oreg., United States of America entitled "Cable TV Measurements Using the 2710 Spectrum Analyzer", at page 12. A spectrum analyzer, such as the 2710 Spectrum Analyzer manufactured by Tektronix, Inc., and a CW signal generator are used, with the CATV system under test receiving either the normal signal source for in-service hum and LFD measurements or a CW signal from the signal generator for measuring hum alone. With the center frequency set to the carrier or test signal frequency in zero span and LINE trigger modes, an operator observes on the analyzer screen the vertical intervals slipping slowly across the screen while line frequency disturbances remain stationary. Amplitude variations of the horizontal sync tips in response to video modulation indicate that the LFD is related to video rather than powerline. The variation of horizontal sync amplitude between vertical intervals in vertical divisions of the screen display may be converted into a percent disturbance. Alternatively with the analyzer in the PEAK acquisition mode the same measurement may be made. This measurement, due to the manual nature and relative expertise of the operator, is imprecise and only gives a single measurement of the hum and LFD.
What is desired is an automatic in-service cable television hum measurement cable technique that filters out the vertical sync energy and gives an accurate time domain sample of the hum and LFD signal present on an in-service, active video channel for precise determination of the percent of disturbance at both the powerline frequency and harmonics thereof.